Potassium channels are transmembrane proteins which are ubiquitously expressed in mammalian cells and represent one of the largest and the most diverse group of ion channels from a molecular perspective. Potassium channels play a key role in regulation of cell membrane potential and modulation of cell excitability. Potassium channels are largely regulated by voltage, cell metabolism, calcium and receptor mediated processes. [Cook, N. S., Trends in Pharmacol. Sciences 1988, 9, 21; and Quast, U., et al., Trends in Pharmacol. Sciences 1989, 10, 431]. Calcium-activated potassium (KCa) channels are a diverse group of ion channels that share a dependence on intracellular calcium ions for activity. The activity of Kca channels is regulated by intracellular [Ca2+], membrane potential and phosphorylation. On the basis of their single-channel conductances in symmetrical K+ solutions, KCa channels are divided into three subclasses: large conductance (BK or Maxi-K) are those having a conductance of greater than about 150 picosemens (pS); intermediate conductance are those having a conductance of about 50–150 pS; and small conductance are those having a conductance of less than about 50 pS. Large-conductance calcium-activated potassium channels are present in many excitable cells including neurons, cardiac cells and various types of smooth muscle cells. [Singer, J. et al., Pflugers Archiv. 1987, 408, 98; Baro, I., et al., Pflugers Archiv. 1989, 414 (Suppl. 1), S168; and Ahmed, F. et al., Br. J. Pharmacol. 1984, 83, 227].
Potassium ions play a dominant role in controlling the resting membrane potential in most excitable cells and maintain the transmembrane voltage near the K+ equilibrium potential (Ek) of about −90 millivolts (mV). It has been shown that opening of potassium channels shift the cell membrane potential towards the Ek, resulting in hyperpolarization of the cell. [Cook, N. S., Trends in Pharmacol. Sciences 1988, 9, 21]. Hyperpolarized cells show a reduced response to potentially damaging depolarizing stimuli. Those BK channels which are regulated by both voltage and intracellular Ca2+ act to limit depolarization and calcium entry and may be particularly effective in blocking damaging stimuli. Therefore cell hyperpolarization via opening of BK channels may result in protection of neuronal cells, as well as other types of cells, e.g., cardiac cells. [Xu, W., Liu, Y., Wang, S., McDonald, T., Van Eyk, J. E., Sidor, A., and O'Rourke, B. Cytoprotective Role of Ca2+-activated K+ Channels in the Cardiac Inner Mitochondrial Membrane. Science 2002, 298, 1029–1033].
BK channels have been shown to be one of two physiologically relevant potassium channels in relaxing human smooth muscle, including both intestinal and penile smooth muscle. Evidence that relaxation of smooth muscle is beneficial to irritible bowel syndrome and erectile dysfunction has been reported. See the following references: Christ, G. J. Drug News Perspect. 2000, 13, 28–36; Poynard T. et al. Alimentary Pharmacology and Therapeutics 2001, 15, 355–361; and Argentieri, T. M. U.S. patent application Ser. No. 2002/0183395.
A variety of synthetic and naturally occurring compounds with BK opening activity have been reported. 4-Aryl-3-hydroxyquinolin-2-one derivatives are disclosed in U.S. Pat. No. 5,892,045, issued Apr. 6, 1999; U.S. Pat. No. 5,922,735, issued Jul. 13, 1999; U.S. Pat. No. 6,353,119, issued Mar. 5, 2002. 4-Arylquinolin-2-one derivatives are disclosed in U.S. Pat. No. 6,184,231, published Feb. 6, 2001 and U.S. Pat. No. 5,972,961, published Oct. 26, 1999. See also Hewawasam, P. et. al. J. Med. Chem. 46 2819–2822 (2003). Other quinolinones are disclosed in U.S. Pat. No. 6,538,022; Japanese Pat. No. 2002371078; and PCT patent application WO 2002026713.
Despite advances in the art, further advances are needed for compounds capable of modulating potassium channels, in particular, large-conductance calcium-activated potassium channels. Such compounds would be useful in treating conditions arising from dysfunction of cellular membrane polarization and conductance.